This invention pertains to press-formed paperboard containers, such as paper plates, paper trays, paper cups and the like. In particular, this inventions pertains to paperboard containers having enhanced rigidity and the method of making such containers.
Many efforts have been made to improve the strength and rigidity of the paperboard containers economically. For example, the inventions disclosed in U.S. Pat. Nos. 4,609,140 and 4,606,496 provided paperboard containers having improved rigidity obtained by optimizing the amount and location of pressure applied during press-forming of the containers. The disclosures of the two prior art patents are incorporated herein by reference to provide disclosure of currently conventional methods of forming press-formed paperboard containers.
In the common process, paperboard web after drying is passed through a size press to apply starch sizing or other binders to both surfaces of the web. After smoothing the surfaces, the web surface that will be used as the top or upper surface of a container, such as a plate or bowl, is coated with at least one functional coating material that forms a barrier to moisture. Such a barrier is intended to prevent moisture from the food or drink placed in the container from wetting the paperboard. Many layers of functional coating material may be placed on the upper surface. In some products, the upper surface may also be printed, the printing then being covered with one or more layers of functional coating material to preclude contact between the printing ink and food placed on or in the container. The coated paperboard is then rolled.
To form paperboard containers from the sized and coated paperboard stock, the paperboard web is moistened on the un-coated lower surface to achieve a predetermined level of moisture in the paperboard. The moisture content aids in deformation of the paperboard during press forming into the desired container shape. The moisture is generally added by a common device known as a flooded nip in amounts sufficient to achieve about 8-12% moisture. Since paperboard when dry contains about 3-6% moisture, the flooded nip can add up to about 6% additional moisture. While water is a preferred moistening medium, some moistening solutions contain wax and surfactant, the former to act as a lubricant in the pressing operation and the latter to assist in moisture penetration. An available concentrate for a moistening solution is VELVETOL(copyright) sold by Rhone-Poulenc of Cranbury, N.J.; it contains polyolefin wax and ethoxylated surfactant.
The moistened paperboard web may be rolled and cured for up to 24 hours after which it is die-cut into blanks having the shape and dimensions appropriate for the container to be made. Each blank is then fed into a die press in which mating dies, heat and pressure work to form the paperboard blank into the desired container.
In evaluating the process for making paperboard containers, the inventor has determined that application of a functional coating material to the lower surface of the paperboard, that is the surface that is not intended to contact food or drink, improved container rigidity without adversely effecting production or quality of the containers. The invention is contrary to generally accepted approaches in the field. As noted above, functional coating material is normally applied to form a barrier to moisture. If moisture-proof functional coating material were applied to both surfaces of a paperboard blank, moisture in the blank that is necessary for press forming would be trapped resulting in blistering of the surface coating or rupture of the paperboard during press forming.
The object of the invention, therefore, is to improve container rigidity by application of a functional coating material to the non-functional surface of the container. In this application, the preferred functional coating used is a diluted latex. Latex is known for its bonding and adhesion characteristics and for plasticity under heat and pressure.
As embodied and broadly described herein, the invention is a container press-formed of paperboard having a top surface disposed for contact with material to be contained and an opposed bottom surface, the container comprising a first functional coating material dispersed in the paperboard proximate the bottom surface without forming a barrier to moisture.
In preferred embodiments, a second functional coating material is disposed on the top surface in quantities sufficient to form a barrier to moisture.
Preferably, the first functional coating material in the bottom surface is the same as the second functional coating material on the top surface. The first functional coating material preferably is a latex. In the presently preferred embodiment, the first functional coating material is TYKOTE(copyright) BASE II distributed by Reichold, Inc. of Research Triangle Park, N.C. TYKOTE(copyright) BASE II is a modified styrene butadiene polymer. The container of the invention preferably comprises first functional coating material at about 0.5 pounds per ream of paperboard, a ream of paperboard being 3000 sq. ft.
The invention further contemplates a method of making the container of the invention comprising the steps of forming the paperboard; applying a solution of water and the first functional coating material to the bottom surface, the amount of first functional coating material in the solution being selected to avoid formation of a barrier to moisture on the bottom surface; cutting a blank for the container from the paperboard; and press-forming the blank at a predetermined temperature and pressure to form the container.
Preferably, the amount of first functional coating material in the moistening solution is selected to avoid blister formation during press-forming of the containers. In a preferred embodiment, the moistening solution includes about 10% solids from the first functional coating material.